Issue #2143405 by chx, bdone: Split migrate into to migrate and migrate_drupal.

...igrate_drupal/config/migrate.migration.d6_system_cron.yml

...drupal/config/migrate.migration.d6_system_performance.yml

...migrate_drupal/config/migrate.migration.d6_system_rss.yml

...igrate_drupal/config/migrate.migration.d6_system_site.yml

...rupal/migrate_drupal/Plugin/migrate/source/D6Variable.php

...igrate_drupal/Plugin/migrate/source/d6/Drupal6SqlBase.php

...ib/Drupal/migrate_drupal/Tests/Dump/Drupal6SystemCron.php

...al/migrate_drupal/Tests/Dump/Drupal6SystemPerformance.php

...lib/Drupal/migrate_drupal/Tests/Dump/Drupal6SystemRss.php

...ib/Drupal/migrate_drupal/Tests/Dump/Drupal6SystemSite.php

...lib/Drupal/migrate_drupal/Tests/MigrateDrupalTestBase.php

.../Drupal/migrate_drupal/Tests/MigrateSystemConfigsTest.php

core/modules/migrate_drupal/migrate_drupal.info.yml

core/modules/migrate_drupal/migrate_drupal.module

...upal/tests/Drupal/migrate_drupal/Tests/D6VariableTest.php